1. Field of the Invention
The present invention relates to a system for monitoring high voltage cables in air stretches.
2. Description of the Related Art
Air lines for power supply is subject to mechanical strains from the environment such as snow and ice load, lightnings, extreme temperatures and wind given from terrain and geographical and topographical conditions. Critical wind strains are extreme wind gusts, vibrations wearing on the end mounts and galloping which may raise from a combination of wind and ice loads and may produce short circuits and damage on the lines. In general extreme climatic strains may result in power breaks and substantial repair works with large economical consequences. A real time monitoring system especially in inaccessible areas may provide information about lines and/or masts being subject to critical loads so that corrective procedures may be performed before larger damage occurs.
Conventional methods for registering climatic strains have until now mainly been based on indirect measurements of the search for variables. Simple ice rigs or a couple of stretches of conductors without voltage is combined with strain detectors to provide a value for icing in the related area. Optical telescopes are used for measuring vibrations over short time periods. Video cameras are used for visual inspection of galloping in chosen point on the line. Common for all these solutions are that they do not measure the direct strain on the line, but variables which in a varying degree is directly related to this. One of the reasons for these problems is the handling at the high voltages. These methods depends on local power supply. Optical fibers of quarts glass are immune to electromagnetic fields and the signal propagation is therefore not affected by the high voltage environment. Also the signals may be transported over long distances so that one does not have to depend on local power supplies. Supply fibers positioned in a suitable fiber optic cable may easily be winded around a power line in the same way as fibers used for communication, which now is a well known method. Alternatively the fibers are integrated in a line strand. The most important difference between the existing and the present method for measuring climatic strains is, however, that the climatically induced strains on the power line is measured directly, and not through variable providing a more or less sufficient basis for calculating them.
The invention consists of a new method making it possible to register climatic strains in high voltage air stretches. The method is based on fiber optic sensors, preferably Bragg gratings. This type of measuring systems are described in a number of different patent publications, for example U.S. Pat. No. 5,845,033, which describes a fiber optic measuring system for monitoring pipes in oil and gas systems. The optical measuring system comprises an optical fiber being wound along a helical line on the pipe. In its longitudinal direction the optical fiber is provided with a number of sensors, being Bragg gratings, adapted to reflect light with different wavelengths. A light source emits light with a large range of wavelengths into the fiber. As the different Bragg gratings reflect light with different wavelengths strain induced changes in the different gratings will indicate the amplitude and the position of the provided strain as changes in the spectrum of the reflected light. This system demands robust solutions because of the very demanding environment in which it is meant to be used, as the sensor is positioned on the outside of the pipe. The system also requires dedicated equipment adapted to the measurements.
In international patent application PCT/US94/00967 a more general description of a known sensor system of the type used in the above mentioned US patent is provided, without giving any specific field of use.
It is an object of the present invention to provide a monitoring system for power lines which in an exact and cost effective way makes it possible to monitor under varying weather conditions without positioning sensor equipment locally. This is obtained with a system and a use of the equipment as described in the independent claims.
Recently fiber optic cables for telecommunication purposes has been mounted on high voltage cables. An additional advantage related to this invention is that existing equipment may be used to mount and to a certain degree control the measuring system, which reduces the costs and provides a cost effective system.
The invention will be described below with reference to the accompanying drawings, illustrating examples of embodiments.